The invention relates to an anaesthetic controller for influencing the rate of active substance supply to a patient""s body to attain and maintain a required state of anaesthesia.
For quite some time research work has been carried out with regard to closed-loop control systems for anaesthesia purposes. In the field of pharmaceutical research fundamental action models of specific active substances have been developed. The findings allow a mathematical simulation of the active substance concentration in the patient""s blood plasma. The prediction accuracy is approximately xc2x130%. Latest applications allow a model-based control of specific active substances. According to the handling habits of the user it is desired to make use of the quality of predicting algorithms or the quality of directly controlling algorithms. An essential feature in this connection is the fundamental finding that an empirically determined model cannot simulate the entire correlations and the calculated concentration is not necessarily connected with the active effect and its intensity. In any case, a medical experience-based correction carried out by the user utilizing various non-measurable parameters for evaluating the intensity of the active effect is desirable.
It is an object of the invention to provide an anaesthetic controller operating on the basis of the active substance concentration in the patient""s body although said active substance concentration is not detectable as a measured variable in real time.
According to the present invention the present active substance concentration in the patient""s body is calculated on the basis of a patient model taking into consideration former active substance supply values. According to the current value of the present active substance concentration calculated in this way the active substance supply rate to the anaesthetic controller is changed such that the present active substance concentration is controlled to attain a target value. In this manner a closed-loop control system controls the active substance concentration in the patient""s body such that a desired target value is attained. The patient model is operated on the basis of a predicting algorithm taking into consideration former active substance supplies, wherein it is possible to distinguish between different distribution periods with different time responses. The patient model thus delivers, on the basis of the preceding active substance rates and supply periods, information on the present active substance concentration in the patient""s body. This calculation is of the predicting type and is termed TOI method (target-oriented infusion). The parameter set required for calculating the active substance concentration is taken from a drug data bank and transferred to the patient model. The patient model calculates the active substance concentration on the basis of the profile of the former active substance supply, wherein this value can be used as actual value for control purposes.
The target value of the active substance concentration can be adjusted by the user on an input device. This allows medical knowledge to be integrated in the control. The supervising physician can determine a target value on the basis of a time chart of the active substance concentration in the patient""s body and adjust said target value. He can also carry out a time modification of said target value.
According to a preferred aspect of the invention the control portion for the active substance concentration is supplemented by a BIS control portion. The latter delivers a respective target value of the active substance concentration from the bispectral index. Said bispectral index is a characteristic value derived from the patient""s EEG and indicates an anaesthestic level. The index ranges between values of 0 (wide awake) and 100 (sedated). On the basis of the respective BIS value the target value of the active substance concentration can be manually adjusted. However it is also possible to use the BIS value for automatic adjustment of the target value of the active substance concentration. Preferably the time behaviour of the BIS value is indicated on a display. The user can thus obtain at any time a complete representation of the active substance supply, the active substance concentration and the depth of anaesthesia.